1. Field
The present invention relates generally to the field of wireless communication systems, and more specifically to dual-mode communications, which support both analog and digital standards, such as CDMA.
2. Background
Digital wireless communication systems, such as code division multiple access (“CDMA”) systems, typically have what is referred to as “dual-mode” capability. A dual-mode handset is a portable telephone handset that supports two types of services. It can switch from analog to digital or from cellular to satellite or from cellular to cordless. For example, a dual-mode handset, also referred to as a “mobile station” in the present application, can incorporate both CDMA and Advanced Mobile Phone Service (“AMPS”) services. Dual-mode capability can be required, for example, for roaming which is the ability to use a communications device such as a cellphone or mobile station and be able to move from one cell or access point to another without losing the connection.
AMPS phone service belongs to “first generation” cellular technology. Digital systems, such as CDMA systems, belong to “second generation” digital technology. In CDMA systems each user uniquely encodes its communication signal into a transmission signal in order to separate its signal from those of other users. The encoding of the message signal spreads its spectrum so that the bandwidth of the encoded transmission signal is much greater than the original bandwidth of the message signal. For this reason CDMA systems are also referred to as “spread spectrum” systems. In digital systems each user communicates with other users through a base station, typically located in the center of each cell. It is possible for a user's communication signal to return to the user, after propagating through the communication network, in the form of a network echo, due to the network transmission delay in the propagation of the communication signal. The network echo can be disruptive to normal conversation. To counteract the effects of network echo, digital systems, such as CDMA systems, typically provide a mechanism in the base station to detect and attenuate network echo.
In a typical AMPS call, the network transmission delay is short enough that any network echo that finds its way back to the mobile station does not generally cause any problems. Thus, in general, no mechanism has been provided in AMPS base stations for detecting and attenuating network echo. In a digital FM call, where analog to digital conversion of the voice signal is used to enhance the quality of signal transmission, for example, using a vocoder and pulse code modulation, the network transmission delay is sufficiently large to cause a perceptible network echo, which can disrupt conversation.
In addition, the mobile station, which can use a handset, headset, or speakerphone to interface with the user, typically includes an acoustic echo canceller. The acoustic echo canceller counteracts feedback in the form of sound waves, i.e. acoustic feedback, from the speaker to the microphone in the mobile station. The network echo can have a disruptive effect on the performance of the acoustic echo canceller. For example, feedback interactions between the network echo and the acoustic echo canceller in the mobile station can lead to piercing tone generation referred to as “howling”. Attempting to provide a simple solution by blocking the second party's transmit path while the first party is talking and vice versa can disrupt the natural flow of conversation by, for example, preventing the second party from interrupting or responding to the first party while the first party continues to speak.
When roaming, the dual-mode mobile station can be in either mode. It is therefore necessary, for maximizing the user's convenience, to be able to provide the same features in either mode. For example, if a hands free speaker phone feature were to be provided in CDMA mode but not in AMPS mode due to the effects of network echo, the user could experience annoying difficulties in trying to use the hands free speaker phone feature while roaming.
Thus, there is a need in the art for detecting and attenuating network echo for first generation technology in dual-mode wireless communication systems. Moreover, there is a need in the art for detecting and attenuating network echo which interacts with acoustic echo cancellers present in dual-mode mobile stations.